1. Field of the Invention
This invention relates in general to Magneto-Resistive (MR) head controllers, and more particularly, to MR head controllers that reduce the time required to transition from read mode to write mode.
2. Description of Related Art
Capacities for hard disk drives are increasing faster than ever before. Whether the hard disk application is serving network servers, desktop workstations or notebook systems, hard disk memory capacity is pressed to meet consumer demand. Lower cost per megabyte (MB), in addition to more memory, is also highly desirable. Adding disks and heads, therefore, becomes less desirable, yielding to the push for higher efficiency. Increasing areal densities, or bits of data per square inch of media, becomes the engineering challenge to get higher efficiency.
Magnetic disk drive manufacturers to date have successfully doubled capacities of the magnetic disk drives every 12 to 18 months, by increasing areal density. As areal densities increase, however, smaller recorded patterns result, which yields weaker signals being generated by the read head. Various techniques exist that can compensate for the weaker signals, such as allowing the read head to fly closer to the magnetic medium, or magnetic disk. Other methods such as improved read channel performance or improved read element design are effective to compensate for the weaker read signals, but these techniques are soon reaching the limits of usefulness.
The transition from the inductive head technology used since the inception of the hard disk, to that of Magneto-Resistive (MR) heads has continued the pace of areal density improvements seen today. MR heads can deliver up to four times the areal density possible with conventional thin film inductive heads. The MR technology permits the continued reduction in cost of magnetic storage technology and has several advantages over thin film inductive heads including, separate read and write elements, high signal output, low noise and velocity independent output.
One characteristic of the MR head technology, is that the MR head incorporates separate read and write elements. A thin film inductive element for recording data onto the magnetic media and a magnetically sensitive resistive element for detecting data bits written to the magnetic media. Each of the two elements can be optimized to perform its particular function. The number of wire turns associated with the write element, for example, can be as low as ten, resulting in a low level of write inductance, which increases the data frequency allowed for write operations. In addition, the read element of the MR head can be made narrower than the written track, such that some misalignment between the read head and the track can be tolerated.
The write element of an MR head generally consists of two magnetic pole pieces that are typically made of a soft magnetic material, such as permalloy. The pole pieces are connected at the ends that point away from the disk surface. Coiled through the middle of the pole pieces is a deposited copper wire. When a current is conducted through the coil, it produces a magnetic field that jumps across the gap between the two inner ends of the pole pieces at the surface of the head. The magnetic fringe field associated with the gap is used to write data onto a magnetic medium by reversing the direction of the magnetic fields on the magnetic medium's surface. During a write operation, a pre-amplifier is used to generate the current pulse conducted by the coil, which is generally on the order of 40-100 milliamps (mA).
The MR head's separate read element consists of a stripe of permalloy material, or MR stripe, placed next to one of the write element's magnetic pole pieces. The electrical resistance of the MR stripe changes by a few percent when it is placed in the vicinity of a magnetic field. The change in electrical resistance of the MR stripe allows the MR head to detect magnetic flux transitions associated with recorded bit patterns. During a read operation, the read pre-amplifier generates a small bias current through the MR stripe, which sets up a bias potential across the MR stripe. As the MR stripe is exposed to the magnetic field from the magnetic medium, the resistance of the MR stripe changes and a voltage change is measured across the MR stripe as a result of the resistance change caused by the magnetic field. A read signal is then generated, which is proportional to the change in bias potential caused by the magnetic field.
As the MR head flies over the magnetic medium, various read and write operations are performed on the magnetic medium. A hard disk application within a computer system, for example, uses sectors within the hard disk to store data for future access. Circuitry controlling the MR head, causes the MR head to alternate between write and read operations, according to whether the MR head is storing data to the hard disk or retrieving data from the hard disk, respectively. The read and write operations generally toggling at the boundaries to the sectors.
Since the MR head contains separate read and write elements, separate read and write data channels along with the associated control are required as well. In read mode, for example, a read channel preamplifier is situated between the MR read element and the read channel so as to amplify the detected magnetic data prior to transmission to the read data channel. Likewise, an amplifier circuit is used in write mode, such that the write amplifier is fixed between the write data channel and the MR head's write element. The write amplifier is required to supply a relatively large amount of current to the write element of the MR head and generally requires a finite amount of time to stabilize, i.e.—an amount of time required before the current supplied by the write amplifier is considered to be reliable.
A problem exists, which is inherent with current generation write electronics, such that the transition between the two modes of operation of the MR head causes unstable current behavior in the write element of the MR head, especially when the MR head transitions from a read to a write operation. A warm up time, or stabilization period, is required before the write channel amplifier can be used for write operations. During the stabilization period, inaccurate and uncontrolled currents may be flowing through the write coil of the MR head, which in turn may cause damage or erasure of pre-existing data to that portion of the hard disk that the MR write head is positioned over. To avoid damage to data that may be stored on the magnetic disk, blank areas on the magnetic disk must be predefined to account for the stabilization period required for the transition between read and write operations. Blank areas, however, subtract from the usable area on the hard disk, which decreases the storage capacity of the hard disk. A direct correlation between reduced storage capacity and write channel stabilization results, whereby any decrease in the stabilization time required prior to the write operation results in a reduced blank area requirement for the hard disk and thus, increases the storage capacity of the hard disk.
A need exists, therefore, for a method and apparatus that is effective to reduce the stabilization time required prior to the write operation of the MR head. A reduction in the stabilization time required would directly result in an effective increase in the areal density of the hard disk and would support the current effort to improve areal densities of magnetic media.